1. Field of the Invention
This invention relates to abrasive grains, and, more particularly, abrasive grains having a particular shape and size.
2. Discussion of the Prior Art
Three basic technologies that can be employed in the production of shaped abrasive materials suitable for use as abrasive grains are fusion, sintering, and chemical ceramic.
In general, abrasive grains prepared by means of a fusion process can be shaped by a chill roll with or without an engraved roll face, a mold into which molten material is poured, or a heat sink material immersed into an aluminum oxide melt. U.S. Pat. No. 3,377,660 discloses an apparatus utilizing a trough for flowing molten abrasive material from a furnace onto a cool rotating casting cylinder, rapidly solidifying it into a thin semi-solid curved sheet, densifying the semi-solid material by reversing its curvature by pulling it away from the cylinder with a driven rapidly cooled conveyor means whereupon the partially fractured strip is deposited onto collecting means in the form of large fragments which upon being rapidly cooled and solidified break up into smaller fragments well adapted to be reduced in size to form conventional abrasive particles. U.S. Pat. Nos. 4,073,096 and 4,194,887 disclose a process for the manufacture of abrasive material in which (1) an abrasive mix is brought to fusion as in an electric arc furnace, (2) a relatively cold substrate is dipped into the molten material whereby a layer of solid abrasive material is quickly frozen (or plated) on the substrate, (3) the plated substrate is withdrawn from the molten material, and (4) the solidified abrasive material is broken away from the substrate and collected for further processing to produce abrasive grains.
Abrasive minerals prepared by a sintering process can be formed from refractory powders having a particle size of 1 to 10 micrometers in diameter. Binders can be added to the powders along with a lubricant and a suitable solvent, e.g., water. The resulting mixtures, pastes, or slurries can be shaped into platelets or rods of various lengths and diameters. In firing the resulting shaped grains to density, high temperatures, e.g. 1400.degree. to 180.degree. C., high pressures, and long soak times, up to 10 hours, must be used. Crystal size may range from under one micrometer up to 25 micrometers. To obtain shorter residence times and/or smaller crystal size, either the pressure or temperature must be increased. U.S. Pat. No. 3,079,242 discloses a method of making abrasive grain from calcined bauxite material comprising reducing the material to a fine powder, compacting under affirmative pressure and forming the fine particles of said powder into grain sized agglomerations, and sintering said agglomerations of particles at a temperature below the fusion temperature of the bauxite to induce limited recrystallization of said particles, whereby to produce abrasive grains directly to size. U.S. Pat. No. 4,252,544 discloses alumina abrasive grains wherein the grain structure is constructed of electrofused or high temperature calcined alumina coarse crystal particles and alumina fine crystal particles which are located between said alumina coarse crystal particles, the latter having particle sizes smaller than that of the former and being processed at a calcination temperature lower than that of the former, wherein said fine crystal particles are sintered integrally with said coarse crystal particles. U.S. Pat. No. 3,491,492 discloses a process for making an aluminous abrasive grain formed from bauxite, or mixtures of bauxite and Bayer process alumina wherein the comminuted aluminous material is mixed with water and ferric ammonium citrate, or with ferric ammonium citrate and citric acid and reduced to a state of fine subdivision by milling to give a fluid slurry of high solid content, drying said slurry to coherent cakes having a thickness equal to one dimension of the final grain before sintering, breaking said cakes to grains, screening, optionally rounding said grains by air mulling, screening, sintering, cooling, and screening to yield the final product. U.S. Pat. No. 3,637,630 discloses a process in which the same aluminous slurry disclosed in U.S. Pat. No. 3,491,492 is plated or coated on a rotating anode of an electrolytic cell. The wet, high density platelets of aluminous material are removed from the rotating anode via air impingement, dried, crushed, sized and sintered to density.
Chemical ceramic technology involves converting a colloidal dispersion or hydrosol (sometimes called a sol) in a mixture with solutions or other sol precursors to a gel or any other physical state which restrains the mobility of the components, drying, and firing to obtain a ceramic material. A sol can be prepared by precipitation of a metal hydroxide from an aqueous solution followed by peptization, dialysis of anions from a solution of metal salt, solvent extraction of an anion from a solution of a metal salt, hydrothermal decomposition of a solution of a metal salt having a voiatile anion, etc. The sol contains metal oxide or precursor thereof and is transformed to a semi-rigid solid state of limited mobility such as a gel by e.g., partial extraction of the solvent. Chemical ceramic technology has been employed to produce ceramic materials such as fibers, films, flakes, and microspheres. U.S. Pat. No. 4,314,827 discloses synthetic, non-fused aluminum oxide-based abrasive mineral having a microcrystalline structure of randomly oriented crystallites comprising a dominant continuous phase of alpha-alumina and a secondary phase.
Although the foregoing references, and others, allude to shaped abrasive grains, none have indicated that any particular shape or size would provide enhanced properties with respect to abrading.